This invention relates to a nonaqueous-electrolyte secondary cell comprising a negative electrode, a positive electrode and a nonaqueous-electrolyte, in which at least one of the negative and positive electrodes contains a binding agent and an active-material carrier or active material.
The remarkable progress of electronics technology in recent years has been realizing more smaller and lighter-weight electronic apparatuses one after the other. Accompanying the achievement, there has been a demand for cells or batteries, for use as a portable power supply, which are more smaller in size, higher in weight and higher in energy density.
Hitherto, secondary cells or batteries for general use have mainly been those based on aqueous solution system, such as lead batteries and nickel-cadmium (Ni-Cd) batteries. These batteries, though having excellent cycle characteristics, are not quite satisfactory as to battery weight or energy density.
Recently, many researches and developments have been made of secondary cells using a nonaqueous liquid electrolyte and using lithium or a lithium alloy as the negative electrode, as a substitute for the lead cells and nickel-cadmium cells which are unsatisfactory with respect to battery weight or energy density.
The nonaqueous-electrolyte cells have the excellent features of high energy density, little self-discharge and small weight. This type of cells, however, have the drawback that as the charge-discharge cycle is repeated, crystals of lithium will grow in dendritic form at the negative electrode at the time of discharge and the dendritic crystals will reach the positive electrode, probably resulting in internal short-circuit. This drawback has been a major hindrance to putting the nonaqueous-electrolyte cells into practical use.
In nonaqueous liquid electrolyte secondary cells using a carbon material as a carrier for a negative electrode-active material at the negative electrode, on the other hand, lithium preliminarily carried on the carbon material by a chemical or physical method, lithium contained in the crystal structure of a positive electrode-active material and lithium dissolved in the liquid electrolyte are each doped into portions between carbon layers at the negative electrode and released from the portions, at the times of charging and discharging. Therefore, repetition of the charge-discharge cycle will not cause the deposition of dendritic crystals on the negative electrode at the time of charging Thus, this type of secondary cells will hardly suffer internal short-circuit, and will exhibit good charge-discharge cycle characteristics. These secondary cells also have high energy density and small weight, and developments are in progress toward practical use of the cells.
Applications for the nonaqueous liquid electrolyte secondary cells as mentioned above include video cameras, lap-top personal computers, etc. Because most of these electronic apparatuses consume comparatively large quantities of electric power, the cells or batteries for such use should be able to endure heavy loads.
Therefore, an effective construction for such cells is a spirally wound electrode body structure formed by coiling a web form positive electrode and a web form negative along their longitudinal direction, together with a web form separator sandwiched therebetween. The cells of the wound electrode body structure can have large electrode areas and can therefore endure heavy-load uses.
In the wound electrode body as above, it is desirable to make the electrodes thinner, so as to attain larger electrode areas and pack a larger amount of the active material or active-material carrier in a limited space. For this purpose, it is desirable that the web form electrodes be produced by a process using a paste (or slurry). The process comprises the steps of mixing a binding agent, an active material (or an active-material carrier) and the like to prepare an electrode mix, dispersing the electrode mix in a solvent to obtain an electrode mix slurry, applying the slurry to an electrode collector, and drying the applied slurry to form an electrode mix layer on the electrode collector. According to the process, it is possible to form the electrode mix layer in the web form electrode in a thickness of several micrometers to several hundreds of micrometers.
In order to provide a secondary cell or battery showing excellent performance for a long time when used as a power supply for electronic apparatus as mentioned above, it is necessary to minimize the lowering in capacity attendant on the repetition of the charge-discharge cycle.
With respect to the capacity, the nonaqueous liquid electrolyte secondary cells according to the prior art have not necessarily had satisfactory performance.
As the binding agent in the electrode mix, polyvinylidene fluoride (PVDF) is preferred in view of its good solubility in solvents and its ability to offer excellent performance by being used in a comparatively small amount. However, the drying temperature for the electrode mix slurry, containing the PVDF as the binding agent, has been set comparatively high (e.g., 170.degree. to 180.degree. C. or above) in order to remove as rapidly and effectively as possible the solvent used to prepare the slurry.